Types of Flu

“Seasonal flus change slightly from year to year,” explains Dr. Anthony S. Fauci, director of NIH’s National Institute of Allergy and Infectious Diseases. Once your immune system encounters a virus, it learns to recognize and block it, so it won’t make you sick again. Each year, seasonal influenza viruses change a little bit to evade your immune system. This slow “drifting” from year to year can go on for decades.

Pandemic flu comes rarely—only 3 times in the 20th century. Instead of a little drift, it’s caused by a sudden major shift. That can happen when a virus jumps from an animal to humans. Most people have no immunity to the new virus, since our immune systems haven’t seen anything like it before.

Type A influenza viruses may infect humans (human influenza) and a large variety of animals including pigs (swine influenza) and birds and poultry (avian influenza). Influenza viruses commonly affect the respiratory tract and the usual way of transmission is through direct contact or close proximity with affected individuals or animals.

Influenza is caused by infection of the respiratory tract with influenza viruses, which are classified into 3 types: A, B, and C. Only virus types A and B commonly cause illness in humans.

Influenza A viruses are further classified into subtypes based on 2 surface proteins, hemagglutinin (H) and neuraminidase (N).

Although 3 types and subtypes of influenza virus cocirculate in humans worldwide (influenza A(H1N1)pdm09, A (H3N2), and influenza B viruses), the distribution of these viruses can vary from year to year and between geographic areas and time of year.

Information on circulating virus strains in various regions can be found via CDC (www.cdc.gov/flu/weekly/) or the World Health Organization (www.who.int/topics/influenza/en/).

Avian and swine influenza viruses can occasionally infect and cause disease in humans, usually associated with close exposure to infected animal populations.

Notably, avian influenza A (H5N1) and A (H7N9) have led to sporadic human cases in recent years, and swine-origin A (H3N2) variant viruses have been associated with disease in the United States.Source:CDC Yellow Book 20163

There are three types of influenza viruses with the ability to infect humans. They are designated A, B and C. Only influenza types A and B cause epidemics of illness that typically occur every winter. Influenza type C usually does not cause symptoms or may cause a very mild respiratory illness. This type does not have the severe public health impact of types A and B. Efforts to control the impact of influenza are directed at types A and B.

Currently, different strains circulate globally: three type A viruses and one type B. Type A viruses are divided into subtypes based on differences in two viral proteins called hemagglutinin (H) and neuraminidase (N). The current subtypes of influenza A are designated A(H2N1) and A(H3N2).

Source:MedLinePlus Magazine (NIH)5

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Influenza can be caused by different strains of the influenza virus. (Symptoms for different types of flu are the same.)